1. Field of the Invention
The present invention relates to a vehicle headlamp.
2. Related Art
To a vehicle such as an automobile, together with a headlamp forming high beam and low beam, a fog lamp can be attached as an auxiliary headlamp. The fog lamp is a lamp that does not illuminate a distant place several meters or more ahead like the headlamp forming high beam and low beam, but irradiates a comparatively wide range near the vehicle. Under a condition of poor visibility such as in a dense fog, the fog lamp improves visibility from another vehicle or a walker.
The fog lamp generally includes a light source bulb that emits light, a reflector that reflects the light from the light source bulb forward, and a cover lens that changes the traveling direction of the light from the reflector according to luminous distribution characteristic by refraction.
Further, disclosed in JP-A-07-014404 is an another type of a fog lamp, in which the reflection surface of the reflector is divided into plural parts, and the luminous distribution of the fog lamp is determined by only the reflector in place of the cover lens, whereby any optical characteristic is not given to the cover lens.
Usually, in the vehicle headlamp requiring attention to the luminous distribution characteristic such as the fog lamp, a light source bulb in which a secondary light source is not produced is used. The secondary light source means an imaginary light source formed as follows: the light emitted from the light source such as a filament in the light source bulb reflects on the inner surface and the outer surface of a glass tube of the light source bulb, and the reflection light is collected at a region in the light source bulb. In the secondary light source, a practical light emission matter does not exist. However, this becomes equal, by emission of the light collected on the secondary light source from the light source bulb, to the case where a light source exists in the position where the secondary light source exists.
Next, problems in case that the light source bulb in which such the secondary light source is formed is used will be described.
FIGS. 8A and 8B are diagrams showing images of the filament projected by the fog lamp, in which FIG. 8A shows a case where a light source bulb in which the secondary light source is not produced is used, and FIG. 8B shows a case where a light source bulb in which the secondary light source is produced is used.
Regarding the fog lamp, as shown in FIG. 8A, a position of a cut-off line is previously supposed, and a reflection surface of the reflector and cut of the cover lens are designed so that an end of the image of the filament is projected along this cut-off line. Thus, by projecting the end of the image of the filament along this cut-off line, a brightness-and-darkness boundary can be formed in FIG. 8A, which distinguishes between the brightness and the darkness at the upper and lower sides of the cut-off line.
However, in case that the light source bulb in which the secondary light source is produced is used, the light reflected by the upper half of the reflection surface of the reflector, as shown in FIG. 8B, leaks from an H-line to the upper portion though the cut-off line is supposed on the H-line and the reflection surface of the reflector and the cut of the cover lens are designed.
This is because in the light source bulb in which the secondary light source is produced, the length of the filament becomes practically long due to the secondary light source. Therefore, as shown in FIG. 9, in case that the reflection surface of the reflector and the cut of the cover lens are designed so that B-light from the rear end of the filament is irradiated along the cut-off line, C-light that has incident onto the upper portion of the reflection surface from the rear end of the secondary light source becomes larger than the B-light emitted from the rear end of the filament in incident angle on the reflection surface. In result, the C-light becomes larger also in reflection angle. Therefore, the C-light is irradiated more upward than the B-light emitted from the rear end of the filament.
Therefore, in case that the light source bulb in which the secondary light source is produced is used, the image of the secondary light source shown by a dashed line in FIG. 8B leaks above the supposed cut-off line, and the up-and-down boundary of the cut-off line becomes dim, so that clear brightness-and-darkness boundary cannot be formed.
Therefore, conventionally, the light source bulb in which the secondary light source is produced is not used in the vehicle headlamp such as the fog lamp.